it is not obvious that you can upgrade matplotlib. Much of matplotlib is c code, and version 1.3 to 1.5 had changes to thousands of lines of c. While you might get some bits to work, it seems unlikely it will behave as 1.5 is supposed to.

No, you can only install pure Python modules yourself. Modules such as numpy and matplotlib that use C, Cython, Fortran or other compiled languages need to be integrated into Pythonista by omz, because Apple's app guidelines do not allow compiling native code in an app, and simply because there are no C compilers for iOS on iOS.